Docker: the Linux container engine
 ==================================

 Docker is an open source project to pack, ship and run any application
 as a lightweight container

 Docker containers are both *hardware-agnostic* and *platform-agnostic*.
 This means that they can run anywhere, from your laptop to the largest
 EC2 compute instance and everything in between - and they don't require
 that you use a particular language, framework or packaging system. That
 makes them great building blocks for deploying and scaling web apps,
 databases and backend services without depending on a particular stack
 or provider.

 Docker is an open-source implementation of the deployment engine which

 powers [dotCloud](http://dotcloud.com), a popular Platform-as-a-Service.
 It benefits directly from the experience accumulated over several years
 of large-scale operation and support of hundreds of thousands of
 applications and databases.

 ![Docker L](docs/theme/mkdocs/images/docker-logo-compressed.png "Docker")

 ## Security Disclosure
 

 Security is very important to us.  If you have any issue regarding security, 
 please disclose the information responsibly by sending an email to 
 security@docker.com and not by creating a github issue.

 ## Better than VMs

 A common method for distributing applications and sandboxing their

 execution is to use virtual machines, or VMs. Typical VM formats are

 VMWare's vmdk, Oracle Virtualbox's vdi, and Amazon EC2's ami. In theory
 these formats should allow every developer to automatically package
 their application into a "machine" for easy distribution and deployment.
 In practice, that almost never happens, for a few reasons:

 
   * *Size*: VMs are very large which makes them impractical to store
      and transfer.
   * *Performance*: running VMs consumes significant CPU and memory,
     which makes them impractical in many scenarios, for example local
     development of multi-tier applications, and large-scale deployment
     of cpu and memory-intensive applications on large numbers of
     machines.
   * *Portability*: competing VM environments don't play well with each
      other. Although conversion tools do exist, they are limited and
      add even more overhead.
   * *Hardware-centric*: VMs were designed with machine operators in
     mind, not software developers. As a result, they offer very
     limited tooling for what developers need most: building, testing
     and running their software. For example, VMs offer no facilities
     for application versioning, monitoring, configuration, logging or
     service discovery.
 
 By contrast, Docker relies on a different sandboxing method known as

 *containerization*. Unlike traditional virtualization, containerization
 takes place at the kernel level. Most modern operating system kernels
 now support the primitives necessary for containerization, including
 Linux with [openvz](http://openvz.org),

 [vserver](http://linux-vserver.org) and more recently
 [lxc](http://lxc.sourceforge.net), Solaris with
 [zones](http://docs.oracle.com/cd/E26502_01/html/E29024/preface-1.html#scrolltoc)
 and FreeBSD with
 [Jails](http://www.freebsd.org/doc/handbook/jails.html).
 

 Docker builds on top of these low-level primitives to offer developers a
 portable format and runtime environment that solves all 4 problems.
 Docker containers are small (and their transfer can be optimized with
 layers), they have basically zero memory and cpu overhead, they are
 completely portable and are designed from the ground up with an
 application-centric design.

 The best part: because Docker operates at the OS level, it can still be
 run inside a VM!

 ## Plays well with others

 Docker does not require that you buy into a particular programming
 language, framework, packaging system or configuration language.

 Is your application a Unix process? Does it use files, tcp connections,
 environment variables, standard Unix streams and command-line arguments
 as inputs and outputs? Then Docker can run it.

 Can your application's build be expressed as a sequence of such

 commands? Then Docker can build it.

 ## Escape dependency hell
 

 A common problem for developers is the difficulty of managing all
 their application's dependencies in a simple and automated way.

 
 This is usually difficult for several reasons:
 

   * *Cross-platform dependencies*. Modern applications often depend on
     a combination of system libraries and binaries, language-specific
     packages, framework-specific modules, internal components
     developed for another project, etc. These dependencies live in
     different "worlds" and require different tools - these tools
     typically don't work well with each other, requiring awkward
     custom integrations.
 
   * Conflicting dependencies. Different applications may depend on
     different versions of the same dependency. Packaging tools handle
     these situations with various degrees of ease - but they all
     handle them in different and incompatible ways, which again forces
     the developer to do extra work.

   * Custom dependencies. A developer may need to prepare a custom
     version of their application's dependency. Some packaging systems
     can handle custom versions of a dependency, others can't - and all
     of them handle it differently.

 Docker solves dependency hell by giving the developer a simple way to
 express *all* their application's dependencies in one place, and
 streamline the process of assembling them. If this makes you think of
 [XKCD 927](http://xkcd.com/927/), don't worry. Docker doesn't
 *replace* your favorite packaging systems. It simply orchestrates
 their use in a simple and repeatable way. How does it do that? With
 layers.

 Docker defines a build as running a sequence of Unix commands, one
 after the other, in the same container. Build commands modify the
 contents of the container (usually by installing new files on the
 filesystem), the next command modifies it some more, etc. Since each
 build command inherits the result of the previous commands, the
 *order* in which the commands are executed expresses *dependencies*.

 Here's a typical Docker build process:

 
 ```bash

 FROM ubuntu:12.04
 RUN apt-get update
 RUN apt-get install -q -y python python-pip curl
 RUN curl -L https://github.com/shykes/helloflask/archive/master.tar.gz | tar -xzv
 RUN cd helloflask-master && pip install -r requirements.txt

 ```
 

 Note that Docker doesn't care *how* dependencies are built - as long
 as they can be built by running a Unix command in a container.

 Getting started
 ===============

 Docker can be installed on your local machine as well as servers - both
 bare metal and virtualized.  It is available as a binary on most modern
 Linux systems, or as a VM on Windows, Mac and other systems.

 We also offer an [interactive tutorial](http://www.docker.com/tryit/)
 for quickly learning the basics of using Docker.

 For up-to-date install instructions, see the [Docs](http://docs.docker.com).

 Usage examples
 ==============

 Docker can be used to run short-lived commands, long-running daemons
 (app servers, databases etc.), interactive shell sessions, etc.

 You can find a [list of real-world
 examples](http://docs.docker.io/en/latest/examples/) in the
 documentation.

 Under the hood
 --------------
 
 Under the hood, Docker is built on the following components:
 

 * The
   [cgroup](http://blog.dotcloud.com/kernel-secrets-from-the-paas-garage-part-24-c)
   and
   [namespacing](http://blog.dotcloud.com/under-the-hood-linux-kernels-on-dotcloud-part)
   capabilities of the Linux kernel;

 * The [Go](http://golang.org) programming language.

 Contributing to Docker
 ======================
 

 Want to hack on Docker? Awesome! There are instructions to get you

 started [here](CONTRIBUTING.md).

 They are probably not perfect, please let us know if anything feels
 wrong or incomplete.

 ### Legal
 

 *Brought to you courtesy of our legal counsel. For more context,
 please see the Notice document.*
 

 Use and transfer of Docker may be subject to certain restrictions by the
 United States and other governments.  
 It is your responsibility to ensure that your use and/or transfer does not
 violate applicable laws. 

 For more information, please see http://www.bis.doc.gov

 
 
 Licensing
 =========
 Docker is licensed under the Apache License, Version 2.0. See LICENSE for full license text.